In anticipation of programatic needs, we have been developing in vitro methods for measuring neutralization of hepatitis C virus. Hepatitis C virus vaccines have already been proposed. In order to easily evaluate them, an in vitro assay of neutralization will be necessary as opposed to testing in the chimpanzee, model. Hepatitis C virus grows poorly in cell culture and its replication is difficult to measure. We have passaged one strain of HCV (strain LB, genotype 1b) for more than one year in an attempt to adapt it to growth in cell culture. We have also been working on improved methods for detection and quantitation.. We have optimized the TaqMan quantitative PCR system for HCV in cell culture. Using this system, we can follow growth kinetics and determine if antibody alters those kinetics as a measure of neutralization.